The present invention relates to a laser source used in optical measurement or the like.
A structure of a conventional laser source is shown in FIG. 7. The composition and operation of this laser source are described below. (See G. Tohmon, K. Yamamoto and T. Taniuchi: Proc. SPIE Vol. 898, Miniature Optics and Lasers, 1988.) In the diagram, numeral 9 is a lens barrel made of aluminum; 6 is a frequency doubler having an optical waveguide 7 formed on a LiNbO.sub.3 substrate adhered with resin to the end of the lens barrel 9; 1 is a laser diode, for generating a laser beam 2 with a wavelength of 0.84 .mu.m, attached to the end of the lens barrel 9 opposite the frequency doubler 6; 3 is a collimating lens located on the side of the laser diode 1 from which the laser beam exits; 5 is a focusing lens positioned at the light input side of the frequency doubler 6; 4 is a half-wave plate positioned between the collimating lens 3 and the focusing lens 5; and 8 is an output laser beam emitted from the frequency doubler 6.
The operation of the conventional laser source of FIG. 7 is explained as follows. The laser beam 2, with a wavelength of 0.84 .mu.m as emitted from the laser diode 1, enters the collimating lens 3 to form parallel rays. These rays then go into the focusing lens 5 after the deflection direction is corrected by the half-wave plate 4. The laser beam 2 leaving the focusing lens 5 is focused on the light input area of the LiNbO.sub.3 frequency doubler 6 and propagates through the optical waveguide 7. Because the wavelength is converted to a half, the laser beam is delivered from the aluminum lens barrel 9 as an output laser beam 8 with a wavelength of 0.42 .mu.m.
FIG. 8 shows the power variation of the output laser beam 8 when the focusing spot of laser beam 2 deviates along the depth of the optical waveguide 7 as the focusing spot of laser beam 2, which is focused by the focusing lens 5, enters the optical waveguide 7. The focusing spot location defined as zero on the horizontal axis is the location where the power of the output laser beam 8 becomes maximum. That location represents the optimum optical alignment of the focusing spot location.
FIG. 8 indicates that a very precise optical alignment is needed because a mere focusing spot deviation of .+-.0.33 .mu.m causes a 50% power variation in the output laser beam 8.
FIG. 9 shows the environmental temperature characteristic of the output laser beam 8 for the conventional laser source shown in FIG. 7.
According to a study done by the present inventors, for a laser source having the configuration shown in FIG. 9, when the ambient temperature changes by .+-.10.degree. C., the optical axis deviates under the strain caused by the difference in thermal expansion coefficient between the frequency doubler 6 and the aluminum lens barrel 9 and the power of the laser beam 8 decreases by 50% or more. Such an output drop caused by a change of only .+-.10.degree. C. is a serious problem in actual use, and improvement has been required.